Waste treatment and disposal is an important social and economic issue. Industries throughout the world spend large sums of money to reduce the biological hazards of environmental exposure to toxic substances. One particular environmental problem is the removal of toxic metals and radioisotopes from solid or liquid industrial wastes. Such contaminants can be removed from soils, for example, by treating the soil with an acid that dissolves the metals. Acid dissolution is followed by selective precipitation, electrowinning, or solvent extraction. Acid dissolution is unfortunately very nonspecific, and often produces many by-products that can create serious environmental problems in their own right.
An alternative detoxification process is to encapsulate contaminants in a container or insoluble matrix that prevents their entry into the environment. This approach still requires storage of the bulky matrix, and does not allow regeneration or reuse of the contaminants. Hence there is a need for a biologically compatible waste treatment process that efficiently and effectively separates metals from contaminated materials. There is also a need for such a process that is biologically compatible and permits selective regeneration and reuse of the contaminants.
One of the present inventors previously has disclosed that enhanced extraction of metals can be achieved with ionizable crown ethers, such as crown ether carboxylic acids. The inventors have found that these macrocyclic ethers have cavities that can selectively extract lanthanides and actinides by attracting these species with an ionized side chain.
The metal ion is then inserted into the cavity of the macrocycle to form a chelate. Analyst, 114:451-453 (1989) and Anal. Chem 58:3233-3235 (1986). This mechanism of attracting the ion and inserting it in the ring has earned these compounds the name of "lariat crown ethers." In these reports, a crown ether carboxylic acid (sym-dibenzo-16-crown-5-oxyacetic acid) was used to extract lanthanides from aqueous solutions into an organic phase with high efficiency and selectivity. U.S. Pat. No. 4,908,135 similarly discloses separation of secondary and tertiary amines using a different crown ether, while U.S. Pat. No. 4,942,149 shows separation of racemic compounds with yet other crown ethers.
An unrelated solvent extraction method is fluid extraction, such as supercritical fluid extraction. A supercritical fluid is typically one that is gaseous at ambient conditions, but which is maintained at a temperature and pressure above its critical temperature and pres sure. Although materials may p erform as solvents at sub-critical temperatures and pressures, fluids often perform better as solvents at supercritical conditions. Supercritical solvents can be used to ex tract organic materials such as caffeine from coffee beans. U.S. Pat. No. 4,911,941 provides an example of supercritical carbon-dioxide extraction of caffeine in which green coffee beans are moved periodically through an extraction vessel and contacted with continuously flowing supercritical carbon dioxide. U.S. Pat. No. 4,898,673 shows a similar system in which soluble materials are co ntinuously extracted from solids using supercritical carbon dioxide. The soluble solids are circulated in a closed-loop pipeline with the supercritical fluid.
Supercritical extraction of environmental wastes has not previously been suggested. This may be due to the relatively low solubility of metals and other non-organic materials in supercritical fluids. Direct extraction of metal ions by supercritical carbon dioxide, for example, is inefficient because of the weak van der Waals interaction between metal ions and carbon dioxide. This weak interaction has apparently discouraged efforts to perform supercritical fluid extraction of metals from environmental wastes.